A conductive metallic powder such as one of nickel, copper, silver, or the like is appropriate for internal electrodes of multilayered ceramic capacitors, and in particular, nickel powder has attracted much attention for use in this way. In particular, nickel ultrafine powder produced by a dry-type production reaction method is very promising. Accompanied by reduction in size and capacity-increase of capacitors, internal electrodes are required to have thin layers and to have less resistance, development of ultrafine powder having particle diameters of not more than 0.5 μm, as well as particle diameters of not more than 1 μm, is required.
Conventionally, several kinds of methods for production of such ultrafine particle metallic powder have been suggested. For example, in Japanese Unexamined Patent Application Publication No. Sho 59-7765, a method for production of spherical nickel ultrafine powder having an average diameter of from 0.1 to several micrometers, in which solid nickel chloride is heated and vaporized to nickel chloride gas, and hydrogen gas is blown into this gas at high-speed to cause nucleation growth at the chaotic interface, is disclosed.
Furthermore, in Japanese Unexamined Patent Application Publication No. Hei 04-365806, a technique in which solid nickel chloride is evaporated to obtain nickel chloride gas having a partial pressure in a range of from 0.05 to 0.3, and the gas is reduced in the gas phase at 1004° C. to 1453° C., is disclosed. In this method, since the reduction reaction is performed at about 1000° C. or more, metallic powder particles generated may easily aggregate to grow to form secondary particles in the temperature of reduction process or other later processes. As a result, desired ultrafine metallic powder cannot be reliably obtained.
Furthermore, in Japanese Unexamined Patent Application Publication No. Hei 11-350010, a technique in which metallic powder generated by contacting metal chloride gas and reducing gas is contacted with inert gas and rapidly cooled at a rate of 30° C./sec or more to 800° C., to restrain aggregation of metallic powder particles and growth of secondary particles. In this method, aggregation and growth of secondary particles of metallic powder particles can be restrained, and ultrafine metallic powder can be obtained.
However, accompanied by reduction in size and capacity increase of capacitors, thinning of layers and increase in number of layers are required for recent capacitors. Therefore, by the above-mentioned technique disclosed in the publication No. Hei 11-350010, effects to inhibit aggregation of metallic powder particles and growth of secondary particles are not sufficient, and it is difficult to reliably produce metallic powder having uniform particle size and not having coarse particles.
Furthermore, if the metallic powder produced adheres to inner wall of the reducing furnace in which metal chloride gas and reducing gas are contacted or an inner wall of the cooling device in which the metallic powder generated at the reducing process is rapidly cooled, the metallic powder may grow to form coarse particles, or adhered metallic particles may aggregate to form secondary particles to form coarse particles, and the coarse particles may be mixed in a finished product.
As a technique to prevent aggregation and growth of the metallic powder which is adhered on an inner wall of metallic powder producing device, a technique in which production of metallic powder is periodically stopped and adhering material inside the device is removed mechanically, is disclosed in Japanese Unexamined Patent Application Publication No. Hei 5-163513. Furthermore, a production device for metallic magnetic powder having a removing means in which metallic magnetic powder adhered to a reaction container wall of the metallic magnetic powder is removed without opening the reaction container, is disclosed in Japanese Unexamined Patent Application Publication No. Hei 5-247506.
In the publication No. Hei 5-247506, as a method of removing adhering magnetic powder, a method in which adhering powder is blown off by blowing inert gas, a method in which adhering powder is blown and falled by blowing metallic powder or ceramic powder together with the inert gas, a method in which impact is given to a reaction container from the outside, and the like are disclosed.
However, these methods are methods which require periodic stopping during production to remove adhered powder as is similar to the method disclosed in the publication No. Hei 5-163513. Therefore, adhering of the metallic powder on the inner wall of the device during the production cannot be prevented completely, and mixing of coarse particles in the metallic powder product cannot be prevented completely. Since the production must be periodically stopped, deterioration of productivity is unavoidable.